The objectives of this application are to establish the nephron sites and mechanisms of regulation of phosphate reabsorption. Phosphate deprivation, which results in marked phosphate conservation by the kidney, will be studied in rats. To establish the relative responses of proximal and distal nephron sites of phosphate reabsorption, micropuncture techniques will be used for analysis of both superficial and deep nephron function. The segmental responses to parathyroid hormone will be evaluated in the presence and absence of pretreatment with nicotinamide or glucocorticoids, which we have shown to restore the phosphaturic response to parathyroid hormone inrats fed a low phosphate diet. These studies will allow interpretation of changes in phosphate excretion seen at the level of the whole kidney, and have implication for syndromes characterized by resistance to the phosphaturic effect of parathyroid hormone.